The invention relates to an input circuit for a switched-mode power supply, comprising a voltage transformer unit for transforming a mains voltage to the required low voltage in order to supply a consumer with a low voltage. An input of the input circuit is connectable to the mains voltage to rectify the mains voltage, and an output of the input circuit is connectable to the voltage transformer unit to provide a rectified input voltage for the voltage transformer unit.
Modern switched-mode power supplies in the low-power field, e.g. chargers for mobile phones and the like, have been substantially reduced in size by now due to a number of innovations with respect to the components and circuit technology. The dimensions and the weight are of major importance specifically for travelers. Therefore, one focal point in today's advancements is on miniaturizing the function of such a switched-mode power supply to an extent that allows, in the ideal case, its integration into the terminal device. Due to the high cost pressure along with the fulfillment of pertinent safety standards specifically the input wiring on the primary side of such a switched-mode power supply was more and more reduced with respect to size and the number of components and, in known configurations, is nowadays substantially formed of a fuse resistor, a bridge rectifier and a Π-filter.
FIG. 1 shows a circuit according to the prior art with the characteristic values for a 3 W switched-mode power supply. Moreover, the parasitic elements are illustrated. In this input wiring, the resistor Rlimit fulfills several functions: It allows a starting current limitation to values below the maximum surge current Isurge of the four diodes of the rectifier. It fulfills a safety function by permitting a defined release behavior if the power supply breaks down, and it reduces the pulse voltage for the power supply in transients.
Apart from the mechanical dimensions of the rectifier diodes (or a function-compatible bridge rectifier), which are in conflict with a desired miniaturization, the input wiring shown in FIG. 1 has some more disadvantages. On the one hand, the electrical load on the input wiring during the start-up leads to a reduction in reliability and service life. On the other hand, the function of the resistor Rlimit per se is not reversible, so that an ongoing use of the switched-mode power supply is no longer possible even if only a temporary malfunction has occurred. Moreover, the known configuration as shown in FIG. 1 is not at all protected against a short-term overvoltage of the mains, which occurs comparatively often in countries having a poor supply quality when operated by emergency power supplies.
Therefore, it is the object of the present invention to provide an improved input circuit for a switched-mode power supply which, apart from an increased functionality and operational safety, simultaneously permits an advanced miniaturization and simplified manufacturability.